Method for preparing catalyst for reforming methanol

ABSTRACT

In a method of manufacturing a copper-zinc-aluminum-based methanol reforming catalyst by coprecipitation, the methanol reforming catalyst is obtained by using a copper compound, a zinc compound and aluminum hydroxide, mixing them with an alkaline substance to produce a precipitate, calcining the precipitate obtained, and reducing the calcined product. Thus, a method is provided for manufacturing a highly active, highly heat resistant and durable methanol reforming catalyst which can be used for a long time even under reaction conditions of 300° C. and higher.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of manufacturing a methanolreforming catalyst, and more particularly relates to a method ofmanufacturing a highly active and durable catalyst in a technique ofmanufacturing a reforming gas containing hydrogen as a main ingredientby reacting methanol with steam (or steam and air, depending on cases).

2. Description of the Prior Art

It has been well known that methanol is relatively easily reformed, inthe presence of a catalyst, to a gas having hydrogen as a mainingredient. Particularly recently, a steam reforming of methanol hasbeen attracting attention as a hydrogen supplying technique forcomparatively small-scale fuel cells. The steam reforming reaction ofmethanol is expressed by the following formula (A), and its elementaryprocesses in formulae (B) and (C):CH₃OH+H₂O→CO₂+3H₂−11.8 kcal/mol  (A)CH₃OH→CO+2H₂−21.7 kcal/mol  (B)CO+H₂O→CO₂+H₂+9.9 kcal/mol  (C)

As methanol reforming catalysts, precipitation catalysts of e.g. copper,nickel, chrome, zinc and aluminum oxides have been proposed hitherto asdescribed in Japanese Patent Publication (Unexamined) H3-52643 andJapanese Patent Publication (Unexamined) H5-305234, for example. Inthese techniques, aluminum nitrate is mainly used as aluminum materialand is considered desirable for catalyst production.

In contrast to the steam reforming of natural gas, naphtha and so on,the steam reforming of methanol, which does not require additionalequipment for a desulfurizing process or water-gas shift reactionprocess, is attracting widespread attention as a hydrogen supplyingtechnique for mobile fuel cells such as for use on automobiles, and forstationary fuel cells of comparatively small scale. Since adaptabilityto load variations and long-term durability are required in theseapplications, it is essential to improve heat resistance and durabilityof the catalysts.

Methanol reforming catalysts manufactured by a conventionalcoprecipitation method are unsatisfactory with respect to heatresistance, and the catalyst activity thereof reduces continuously whenused for a long time. This drawback is particularly outstanding underconditions of a reaction temperature of about 300° C. or higher, and thedurability of the catalysts at such temperature is extremely low. Thisproblem is particularly outstanding when aluminum nitrate is used as asource of aluminum as described hereinafter on the basis of testresults.

Further, in a reaction in which the steam reforming of methanol iscombined with partial combustion, the reaction temperature may rise evenhigher. Thus it is necessary to develop a catalyst durable for a longperiod of time even when used at a temperature in the order of 300 to400° C.

Thus, the object of the present invention is to provide a method ofmanufacturing a highly active, highly heat resistant and durablemethanol reforming catalyst which can be used for a long time even underreaction conditions of 300° C. and higher.

DISCLOSURE OF THE INVENTION

The characterizing features of a method of manufacturing a methanolreforming catalyst according to the present invention for fulfilling theabove object are the first through fifth features described hereinafter.

The first feature lies in that, in a method of manufacturing acopper-zinc-aluminum-based methanol reforming catalyst bycoprecipitation, a copper compound, a zinc compound and aluminumhydroxide are used and mixed with an alkaline substance by a mixingoperation such as stirring to produce a precipitate, and the precipitateobtained is calcined under a calcination temperature condition higherthan 380° C. and not exceeding 650° C.

SUMMARY OF THE INVENTION

With this feature, also with the method of the present application inwhich a copper-zinc-aluminum-based methanol reforming catalyst isobtained, coprecipitation is employed, however, aluminum hydroxide isused as an aluminum source instead of using aluminum nitrate as used inconventional methods. Further, as in the conventional methods, acopreciptate of copper, zinc and aluminum is obtained, which is thencalcined to produce a predetermined catalyst.

It is considered that, by using aluminum hydroxide as an aluminumsource, as noted above, instead of aluminum nitrate usually used assalt, copper is uniformly dispersed to prevent sintering, and heatresistance and durability are improved.

Further, with regard to calcination temperature, when the temperature is380° C. or below, calcination is insufficient and tends to result inpoor heat resistance and durability, and when higher than 650° C.,calcination may be excessive. Both cases are undesirable.

When the above technique is employed, as the second feature in additionto the first feature, it is preferable to make a compounding ratio ofcopper, zinc and aluminum, in an atomic ratio of metal atoms, 1:03 to10:0.05 to 2.

When the quantity of zinc is too small, copper sintering cannot easilybe prevented with effect, and when the quantity of zinc is too large,the methanol reforming catalyst tends to be poor in performance.

On the other hand, when the quantity of aluminum is too small, Cu—ZnOcomposition cannot be stabilized, and when the quantity of aluminum istoo large, the methanol reforming catalyst tends to be poor inperformance as in the case of zinc quantity.

Further, when reducing the catalyst, as the third feature, the calcinedproduct provided by the calcination in the first feature noted above,preferably, is reduced by hydrogen at 150° C. to, but not exceeding,300° C. As the fourth feature, the calcined product provided by thecalcination in the first feature, preferably, is hydrogen-reduced byhydrogen gas diluted with an inert gas to a hydrogen concentration notexceeding 6%.

In the hydrogen reduction process which is an exothermic reaction,because of the low melting point of copper, the particle diameter tendsto be increased and the surface area decreased by heat. Further,excessive heat may subtly change the porous structure, which results ina major change in property of the methanol reforming catalyst. Thus,when the mixed oxide is reduced by hydrogen, it is preferable to conductthe hydrogen reduction of copper oxide, which is an exothermic reaction,under mild conditions. By selecting the temperature range and hydrogenconcentration noted above, the reduction can be conducted under mildconditions. However, the methanol reforming catalyst manufacturedaccording to the invention is superior in durability to conventionalcatalysts, and therefore is resistant to the heat generated during thereduction process. The reduction can be conducted without so much careas in the conventional methods. This is another advantage of the presentapplication.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a view showing durability test results of methanol reformingcatalysts.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A method of manufacturing a copper-zinc-aluminum-based methanolreforming catalyst (i.e. a methanol reforming catalyst containingcopper, zinc and aluminum oxides as main ingredients) according to thepresent invention can be executed in various ways as describedhereinafter, for example.

A mixed solution, prepared by adding aluminum hydroxide to an aqueoussolution containing a copper compound (e.g. copper nitrate or copperacetate) and a zinc compound (e.g. zinc nitrate or zinc acetate), is,while being stirred, dripped into an aqueous solution of an alkalinesubstance (e.g. sodium carbonate or potassium carbonate) kept at atemperature of about 60° C. to produce a precipitate.

The process of adding the mixed solution for manufacturing theprecipitate can be conducted in a reverse order, that is, copper, zincand aluminum compounds can be added to the solution of the alkalinesubstance. Further, aluminum hydroxide can be added to the solution ofthe alkaline substance in advance, and the solution containing thecopper compound and zinc compound can be mixed therein to produce theprecipitate.

The produced precipitate is sufficiently cleaned with water, thenfiltered and dried. Next, by calcining this at a temperature in therange higher than 380° C. and not exceeding 650° C., a sintered productof copper oxide-zinc oxide-aluminum oxides is obtained. After anadditive (e.g. graphite) is added as necessary, the sintered productobtained is molded into a shape such as of a tablet or an extrusion. Itis preferable that the calcination temperature is in the range higherthan 380° C. and not exceeding 550° C.

The above sintered product has a compounding ratio of copper oxide, zincoxide and aluminum oxide, in an atomic ratio of metal atoms: copper,zinc, aluminum as 1:0.3 to 10:0.05 to 2, preferably about 1:0.6 to 3:0.3to 1.

Next, the mixed oxide resulting from the above processes is subjected toa hydrogen reduction as necessary. In the hydrogen reduction process,because of the low melting point of copper, the particle diameter tendsto be increased and the surface area decreased by heat. Further,excessive heat may subtly change the porous structure, which results ina major change in property of the methanol reforming catalyst. Thus,when the mixed oxide is reduced by hydrogen, it is preferable to conductthe hydrogen reduction of copper oxide, which is an exothermic reaction,under mild conditions.

For example, it is preferable to conduct the reduction in the presenceof hydrogen gas diluted with a gas not involved in the reaction (e.g.nitrogen gas, argon gas or methane gas) to have a hydrogen content of 6%or less, preferably about 0.5 to 4%, by volume, while maintaining thetemperature at about 150 to 300° C. As a gas not involved in thereaction, in particular, an inert gas such as nitrogen gas is suitablefor use.

The copper-zinc-aluminum-based methanol reforming catalyst provided bythe above method has a dense structure consisting of an agglomerate offine particles. Extremely small copper particles are uniformly dispersedover the surfaces of zinc oxide particles, and maintained in a highlyactivated state by a chemical interaction with zinc oxide.

On the other hand, the aluminum oxide is distributed all over, keepingthe copper particles and zinc oxide particles from sintering under theheat, to be in a highly active state. Thus, when these methanolreforming catalysts are used, high catalytic activity can be maintainedfor a long time even at a comparatively high temperature of 300° C. orabove and under high LHSV conditions.

Embodiments of preparing methanol reforming catalysts according to theabove technique, and a comparative example according to a conventionaltechnique will be described hereinafter in the order of manufacturingmethod first and then properties.

1. Preparation of Reforming Catalysts

(Embodiment 1)

Aluminum hydroxide is added to an aqueous solution of sodium carbonateand, while keeping the temperature thereof at 60° C., a mixed solutioncontaining copper nitrate and zinc nitrate, while being stirred, isslowly dripped to form a precipitate. Thereafter the precipitate issufficiently washed, then filtered and dried.

Next, this is calcined at the temperature of 550° C. for three hours, toobtain Catalyst 1 containing copper oxide-zinc oxide-aluminum oxide in amolar ratio of 1:10.2.

(Embodiment 2)

Aluminum hydroxide is added to an aqueous solution of sodium carbonateand, while keeping the temperature thereof at 60° C., a mixed solutioncontaining copper nitrate and zinc nitrate, while being stirred, isslowly dripped to form a precipitate. Thereafter the precipitate issufficiently washed, then filtered and dried.

Next, this is calcined at the temperature of 550° C. for three hours, toobtain Catalyst 2 containing copper oxide-zinc oxide-aluminum oxide in amole ratio of 1:2:0.5.

(Embodiment 3)

In contrast to Embodiment 1, only the precipitate calcinationtemperature is changed to 450° C., to obtain Catalyst 3.

(Embodiment 4)

In contrast to Embodiment 1, only the precipitate calcinationtemperature is changed to 350° C., to obtain Catalyst 4.

COMPARATIVE EXAMPLE 1

A mixed aqueous solution containing copper nitrate, zinc nitrate andaluminum nitrate in a molar ratio of 1:1:0.2, while being stirred, isdripped to an aqueous solution of sodium carbonate kept at thetemperature of about 60° C., to form a precipitate. Thereafter theprecipitate is sufficiently washed, then filtered and dried.

Next, this is calcined at the temperature of 550° C. for three hours toobtain Comparative Catalyst 1.

2. Catalyst Properties

Property Confirmation Test 1

Nitrogen gas containing 2% by volume of hydrogen is passed over 7.6 cceach of the catalysts obtained in Embodiments 1 through 4 andComparative Example 1, and the catalysts are respectively reduced at thetemperature of 250° C. Then a steam reforming reaction of methanol iscarried out by passing a raw material gas with an S/C (molar ratio ofsteam/methanol)=2 through catalyst layers under the conditions of LHSV(methanol feeding rate (Liquid, Howly, Space, Velocity))=5h⁻¹ andreaction pressure being at atmospheric pressure. Table 1 shows thecatalytic activities at 300° C., 350° C. and 400° C. of each catalystobtained (methanol conversion rate, 1−[CH₃OH]/([CO₂]+[CO]+[CH₃OH]),where [ ] shows each ingredient concentration in the gas after thereaction).

TABLE 1 LHSV (methanol feeding rate) = 5 h⁻¹ Methanol conversion rate(%) 300° C. 350° C. 400° C. Catalyst 1 95.1 98.4 100 Catalyst 2 95.2 100100 Catalyst 3 93.9 100 100 Catalyst 4 94.8 100 100 Comparative catalyst1 64.4 89.0 99.7

As a result, the catalysts manufactured by the invention are found toshow higher methanol conversion rates than the comparative catalyst.

Property Confirmation Test 2

Nitrogen gas containing 2% by volume of hydrogen is passed over 7.6 cceach of the catalysts obtained in Embodiments 1 through 4 andComparative Example 1, and the catalysts are respectively reduced at thetemperature of 250° C. Then a steam reforming reaction of methanol iscarried out by passing a raw material gas with an S/C (molar ratio ofsteam/methanol)=2 through catalyst layers under the conditions of LHSV(methanol feeding rate)=5h⁻¹ and reaction pressure being at atmosphericpressure. A durability test is conducted at the reaction temperature of400° C. The results are shown in Table 2 and FIG. 1. In the Table, thetime indicates Hr and the conversion rate indicates %. Further, in theFigure, the horizontal axis represents time while the vertical axisrepresents methanol conversion rates. According to the results,Catalysts 1 through 4, which are objects of this application, show nodecline in the conversion rate, whereas the Comparative Catalyst usingaluminum nitrate as aluminum source shows a distinct decline.

TABLE 2 Catalyst 1 time 0 20 40 65 90 conversion rate 100 100 100 100100 Catalyst 2 time 0 20 45 70 110 135 conversion rate 100 100 100 100100 100 Catalyst 3 time 0 20 60 90 115 140 conversion rate 100 100 100100 100 100 Catalyst 4 time 0 20 60 90 115 conversion rate 100 100 100100 100 Comparative time 0 20 45 65 90 135 catalyst 1 conversion rate99.7 99.4 96.9 94.8 91.7 72.8

INDUSTRIAL UTILITY

According to the present invention, methanol reforming catalysts of highheat resistance and high activity can be obtained. Thus the catalystsprovided have high activity and long life even under the conditions ofhigh reaction temperature and high LHSV (methanol feeding rate).

1. A method of manufacturing a copper-zinc-aluminum-based methanolreforming catalyst by coprecipitation, the method of manufacturing themethanol reforming catalyst consisting essentially of the steps of:providing a copper compound, a zinc compound and aluminum hydroxide;mixing the copper compound, the zinc compound and the aluminum hydroxidewith an alkaline substance to produce a precipitate; and calcinating theprecipitate under a calcination temperature condition higher than 380°C. and not exceeding 650° C.
 2. The method of manufacturing a methanolreforming catalyst as defined in claim 1, wherein a compounding ratio ofcopper, zinc and aluminum is, in an atomic ratio of metal atoms, 1:0.3to 10:0.05 to
 2. 3. The method of manufacturing a methanol reformingcatalyst as defined in claim 1 further including the step of reducing acalcined product obtained by said calcination by hydrogen at 150° C. to,but not exceeding, 300° C.
 4. The method of manufacturing a methanolreforming catalyst as defined in claim 1, further including the step ofhydrogen-reducing a calcined product obtained by said calcination byhydrogen gas diluted with an inert gas to a hydrogen concentration notexceeding 6%.